The present invention relates to a fixing device employing a cylindrical heat roller having on its inner wall a heating resistor layer.
As a heat roller used for a fixing device of an image forming apparatus, those each of them has in its hollow portion a heating source such as an infrared lamp, a halogen lamp or a Nichrome wire have been used commonly.
However, in the heat roller having the structure mentioned above, the roller is heated indirectly through air by radiant heat of a heat source. Therefore, there has been a problem that thermal efficiency is poor and the length of time for the surface of the heat roller to arrive at the prescribed temperature (preheating time) is long.
It has therefore been proposed a method to heat a roller directly by forming a heating resistor layer such as a ceramic heater on a circumferential surface of the heater.
However, the aforesaid heat roller to heat a roller directly is a straight roller wherein the outside diameter of the roller which comes in contact with a recording material is the same as the outside diameter which does not come in contact with a recording material.
In the case of this straight roller, therefore, an area exposed to the open air on each of both end portions of the roller is broader than that on the middle portion of the roller, and an amount of heat radiated from both end portions of the roller is greater accordingly, resulting in ill-balanced temperature gradient in the axial direction of the roller.
Namely, temperatures at both end portions tend to be lower than that on the middle portion of the roller, due to radiation of heat from an outer circumferential surface, an inner circumferential surface and end faces of the roller.
This phenomenon is not only dependent on the surface area of the end faces of the roller but also influenced by an air flow surrounding the roller. This point will be explained concretely as follows.
When a roller is heated and heat is radiated into air, a temperature difference between the temperature inside the roller and that in the open air is caused to generate a convection current. In this case, as shown in FIG. 4, power-supply portion E provided on the end portion of roller R is provided with several holes to form an opening.
Therefore, exchange is made between air inside the roller R heated positively and air surrounding the roller. When the roller R is considered to be divided into both end portions and the middle portion, both end portions of the roller R are subjected to air exchange because they are provided with openings, but the temperature difference is hard to be caused on the middle portion of the roller because of the heat of both end portions adjoining each other, and thereby, air exchange is hard to be made.
Therefore, the temperature difference is caused between both end portions where air exchange is made positively and the middle portion where the air exchange is hardly made.
Namely, when the temperature at both end portions of the roller is set to the reference temperature, the temperature at the middle portion of the roller turns out to be higher than the reference temperature, while when the temperature at the middle portion of the roller is set to the reference temperature, the temperature at the both end portions of the roller turns out to be lower than the reference temperature.
In the case wherein a roller having the structure mentioned above is used for a fixing device of an image forming apparatus, when the temperature at both end portions of the roller is set to the reference temperature, the temperature at the middle portion of the roller turns out to be higher than the reference temperature, resulting in occurrence of hot offset when a narrow recording material is fixed continuously.
On the contrary, when the temperature at the middle portion of the roller is set to the reference temperature, the temperature at the both end portions of the roller turns out to be lower than the reference temperature, resulting in occurrence of under fixing when a broad recording material is fixed continuously.
However, in the case of the aforesaid roller to heat a roller directly, width (WR) of heating resistor layer RL which is formed on a circumferential surface of heat roller HR is set to the maximum width size (maximum width paper (WPmax)) among recording sheets P to be conveyed, as shown in FIG. 11 (a). For example, when recording sheets to be conveyed are composed of four types of A3, A4, B4 and B5, the width of heating resistor layer RL is set to the width of A3 recording sheet representing the maximum width size or greater.
In the fixing device having the structure mentioned above, temperature distribution in the axial direction of heat roller HR in the case of fixing the recording sheet having the maximum width size is inside the range between the upper limit of optimum temperature (UOT) and the lower limit of optimum temperature (LOT) as shown in FIG. 12, though the temperature distribution at both end portions of the roller are lowered because of heat radiation, resulting in satisfactory fixing.
However, when a recording sheet having a small width size is conveyed, the temperature of heating resistor layer RL of non-contact portion (the portion outside the width of the recording sheet) of a heat roller rises, and the surface of heat roller HR is partially deteriorated, reducing the duration of life of the heat roller HR, which is a problem.
When a recording sheet having the maximum width size is conveyed right after the temperature of heating resistor layer RL of non-contact portion rises, high temperature offset is caused because temperatures at both end portions of the heat roller HR are higher than the upper limit of optimum temperature, and deterioration of a fixed image is caused, which is also a problem.